Nanoparticles and Nanostructures Fabricated Using Femtosecond Laser Pulses 11 Fig. 9. Morphological evolution of structures on YBCO thin films induced by linear polarized fs laser with fixed number of pulses N=600,000 and various fluences (a) F = 0 mJ/cm 2 , (b) F = 43 mJ/cm 2 , (c) F = 59 mJ/cm 2 , (d) F = 79 mJ/cm 2 , (e) F = 154 mJ/cm 2 , (f) F = 319 mJ/cm 2 . Inset: 2D Fourier spectra transferred from their corresponding SEM images (10 μm×10 μm with pixel resolution of ~0.04 nm). The scale bar is applied to all pictures. Figures 10(a)-10(c) show the evolution of the ripple structure on YBCO thin films irradiated by a single-beam fs laser with various numbers of pulses (N) and the fixed laser fluence F = 79 mJ/cm 2 . With an increase in the number of pulses, the ripple structure became increasingly clear in SEM images, as evidenced by the appearance of satellite peaks in the 2D Fourier spectra in the insets of Figs. 10(b) and 10(c) [there are no satellite peaks in the inset of Fig. 10(a) for an as-deposited YBCO thin film]. The spatial period of ripples, estimated from the position of satellite peaks in the 2D Fourier spectra, is independent of the number of pulses or irradiation time, as shown in Fig. 11(b). Once the number of pulses ≧ 50,000, i.e. the sample surface was irradiated by the 75 mJ/cm 2 laser pulses for ≧10 s, ripples can be clearly observed on the surface of the sample. In addition, the real-time evolution of the ripple structure appears in the transmission measurements in Fig. 10(d). In the case of F = 154 mJ/cm 2 , the transmission power of the laser beam dramatically increased to within 2 s and then saturated after ~10 s. Some specific points were marked at 79 mJ/cm 2 of Fig. 10(d), and corresponding SEM images are shown in Figs. 10(a), 10(b), and 10(c), respectively. At 0.1 s [i.e. N=500 in Fig. 10(a)], there are almost no structures on the surface of YBCO thin films. However, the rippled structure can be observed at 10 s [i.e. N=50,000 in Fig. 10(b)]; meanwhile, the transmission power dramatically increased due to the thinning of YBCO films inside the grooves. For an extended irradiation time of 30 s [i.e. N=150,000 in Fig. 10(c)], the ripple structure does not change from that of Fig. 10(b), e.g. the spatial period of ripple as shown in Fig. 11(b), except for the contrast of grooves causing slight rise in transmission power in Fig. 10(d). Furthermore, the characteristics of changes in transmission power in Fig. 10(d) are independent of laser fluence. This indicates that the formation of ripple structures is very rapid, with only ~2 s needed, and the formation processes is independent of laser fluence. Laser fluence only affects the spatial period of ripple structures, as shown in Fig. 11(a). Lasers – Applications in Science and Industry 12 Fig. 10. Morphological evolution of structures on YBCO thin films induced by linear polarized fs laser with fixed laser fluence F = 79 mJ/cm 2 and various numbers of pulses (a) N = 500, (b) N = 50,000, (c) N = 150,000. (d) The transmission power of laser pulses as a function of irradiating time, i.e. pulse number N. Inset: 2D Fourier spectra which were transferred from their corresponding SEM images (10 μm×10 μm with pixel resolution of ~0.04 nm). The scale bar is applied to all pictures. Fig. 11. (a) Dependence of the ripple period on the fluence. (b) Dependence of the ripple period on the number of pulses. The dashed lines are a guide to the eyes. Nanoparticles and Nanostructures Fabricated Using Femtosecond Laser Pulses 13 Fig. 12. Morphological evolution of ripple structures on YBCO thin films induced by linear polarized fs laser with F = 300 mJ/cm 2 , N=150,000, and various incident angles (a) θ = 0°, (b) θ = 30°, (c) θ = 60°. (d) Dependence of the ripple period on the incident angle of laser pulses. The dashed lines are a guide to the eyes. All SEM images are 10 μm×10 μm with pixel resolution of ~0.04 nm. On the other hand, with the fluence and pulse number fixed at ~300 mJ/cm 2 and 150,000, respectively, we found that the spatial period decreased with an increase in the incident angle (θ) [see Fig. 12(d)]. However, the observed period of ripple at θ = 0° was significantly smaller than the prediction of Λ=λ/(1+sinθ) (Zhou et al., 1982). In addition, the incident angle-dependent period of ripples on YBCO thin films cannot be described using this simplified scattering model [the solid line in Fig. 12(d)]. Therefore, the influence of surface electromagnetic waves, i.e. surface plasmons (SPs) should be taken into account in the formation of subwavelength ripples (Sakabe et al., 2009; Huang et al., 2009). According to Shimotsuma’s et al. results (Shimotsuma et al.; 2003), femtosecond incident light easily excites plasmons on the surface of various materials. As shown in Fig. 13(c), once the momentum conservation condition for the wave vectors of the linear polarized laser light (K i ), the plasma wave (K p ), and the laser-induced subwavelength periodic surface structures (LIPSS, K L ) is satisfied, such plasmons could couple with the incident light. The interference between the plasmons and the incident light would generate a periodically modulated electron density causing nonuniform melting. After irradiation with a femtosecond laser, the interference ripple was inscribed on the surface of the YBCO thin film. Lasers – Applications in Science and Industry 14 Fig. 13. SEM images (10 μm×10 μm with pixel resolution of ~0.04 nm) of fs LIPSS induced by (a) the left- and (b) right-circularly polarized beams; (c) Schematic of the momentum conservation condition of wave vectors of linear polarized laser light (K i ), plasma wave (K p ), and LIPSS (K L ); (d) Schematic processes of the LIPSS by circularly polarized laser light (K i,C ). The scale bar is applied to all pictures. Interestingly, when we used a circularly polarized beam, the rippled structures were still produced, as shown in Figs. 13(a) and 13(b). The orientation of the ripples was set at -45° and +45° for left and right circularly polarized beams, respectively, with respect to the incident plane of the beam. In both cases, the spatial period was 491 nm, as produced by fs laser pulses with a fluence of 185 mJ/cm 2 and number of pulses set to 150,000. These results show the orientation of rippled structures strongly depend on the polarization-state of incident fs pulses. These results are consistent with the results of Zhao et al. on tungsten (Zhao et al., 2007a, 2007b). In principle, circularly polarized light (K i,c ) can be decomposed to two perpendicular linear-polarization lights (E x and E y ) through retardation of λ/4 in phase, as shown in Fig. 13(d). Linearly polarized light E x and E y can induce the LIPSS K L,x and K L,y , respectively, as long as the momentum conservation condition in Fig. 13(c) is satisfied. Thus, both K L,x and K L,y with phase coherent further cause the K L,c according to the momentum conservation condition of K L,c = K L,x + K L,y . The 45° wave vector of LIPSS, K L,c , is completely consistent with the direction of the satellite peaks in the 2D Fourier spectra [the inset of Fig. 13(a)]. Namely, the orientation of ripples is -45° for left-circularly polarized beams with respect to the incident plane of the beam. Similarly, right-circularly polarized beams induce a +45° orientation of LIPSS, K L,c , according to the momentum conservation condition of K L,c = -K L,x + K L,y consistent with the results in Fig. 13(b). Nanoparticles and Nanostructures Fabricated Using Femtosecond Laser Pulses 15 3.3 Generation of YBCO dot structures To produce dot structures on YBCO thin films, we adopted a dual-beam scheme using the modified Michelson interferometer shown in Fig. 14. The polarization of both beams was individually controlled by two quarter-wave plates before the reflection mirrors in both arms of the dual-beam setup. After the beam splitter in the dual-beam setup, both beams were collinearly and simultaneously focused on the surface of the sample using a convex lens with a focal length of 50-mm. Before generating the YBCO dot structures, we measured the interference patterns between two beams to check the temporal overlap of the two pulses. In the inset of Fig. 14, the interference pattern between the two pulses with parallel polarization can be clearly observed after adjusting the delay in one of the two pulses. The polarization of two pulses was set perpendicularly to each other to eliminate interference patterns and generate the YBCO dot structures. All experiments were performed in air under atmospheric pressure. As shown in Figs. 15(a1)-15(d1), it is surprising that many dots rather than regular ripples appeared on the surface of YBCO thin films using a dual-beam setup with perpendicularly linear polarization. In the case of the dual-beam setup, the K L,x and K L,y without coherence in phase induced by random phase and perpendicularly linear-polarization beams (E x and E y ), respectively, would not satisfy the conservation of momentum of K L,c = ±K L,x + K L,y and be unable to create ±45° wave vector of LIPSS, K L,c as shown in Fig. 13(d). Therefore, the K L,x and K L,y which are perpendicular to each other would lead 2D nonuniform melting and further aggregation to form randomly distributed dots [see the 2D Fourier spectra in the inset of Figs. 15(a2)-15(d2)] due to surface tension. In the case of N = 25,000, the average diameter of dots was approximately 632 nm estimated by the log-normal fitting presented in Fig. 15(a2). An increase in the number of pulses resulted in a marked broadening in the size distribution, although the average size only slightly increased from 632 nm to 844 nm [see Figs. 15(a2)-15(d2)]. For N = 300,000, the size of a part of dots was on the order of micrometers. However, larger dots influence the dot density on the surface of YBCO thin films. For instance, the density of dots increases with the number of pulses ≦150,000. Once the dots grow too large to merge with the nearest neighbors, or even next nearest neighbors, the density of the dots significantly shrank, as shown in Fig. 15(c1). In this manner, the size and density of YBCO dots can be controlled by the numbers of pulses from the fs laser. Fig. 14. Experimental setup for the generation of nanodots on YBCO thin films. Lasers – Applications in Science and Industry 16 Fig. 15. Dot structures on YBCO thin films induced by a dual-beam setup with fluence = 87 mJ/cm 2 and various numbers of pulses (a1) N =25,000, (b1) N =50,000, (c1) N =150,000, (d1) N =300,000. (a2)-(d2) The size distribution corresponds to the SEM images (10 μm×10 μm with pixel resolution of ~0.04 nm) (a1)-(d1), respectively. Solid lines are the log-normal fitting. Inset: 2D Fourier spectra which were transferred from their corresponding SEM images (a1)-(d1), respectively. The scale bar is applied to all pictures. 3.4 Characteristics of YBCO nanostructures To characterize the superconductivity of the ripple structures on YBCO thin films, the area of the ripple structure must be large enough to measure. Thus, the scanning scheme shown in Fig. 8 was adopted to prepare the large-area ripple structures on YBCO thin films. After passing through a variable neutral density filter, the beam was two-dimensionally scanned using a pair of galvanic mirrors with a speed of 7.6 cm/s. The laser beam was focused on the surface of the sample with a spot size of 220 μm using an f-theta lens. All experiments were performed in air under atmospheric pressure. It is evident from Fig. 16(g) that the quality of the crystalline structure of the YBCO films remained high after irradiation by the femtosecond laser with fluence up to 260 mJ/cm 2 . However, the quality deteriorated considerably with a further increase in laser fluences. For instance, with an irradiation fluence of 530 mJ/cm 2 , the intensity of the characteristic X-ray diffraction peaks diminished considerably. As shown in Fig. 17, while the superconductivity of the YBCO films remained nearly unchanged under low fluence irradiation, it began degrading at irradiation levels of 320 mJ/cm 2 and disappeared at 530 mJ/cm 2 , indicating structural and compositional changes with higher irradiation fluence. Nanoparticles and Nanostructures Fabricated Using Femtosecond Laser Pulses 17 As mentioned above, the crystalline structure of these YBCO nanodots induced by the laser irradiation (260 mJ/cm 2 ) remained oriented with the c-axis, with sharp diamagnetic Meissner effect characteristics at 89.7 K (Fig. 17), indicating that even after the dramatic morphological reconstruction, the obtained nanodots maintained most of their intrinsic properties. Indeed, as indicated by the energy dispersive spectroscopy (EDS) spectrum displayed in Fig. 16(h), which was taken on one of the nanodots [marked as area 1 in Fig. 16(e)], the composition of the nanodot had not changed from that of the original YBCO films. EDS results taken in the area between the dots [marked as area 2 in Fig. 16(e)] indicates no signal of Ba. Instead, traces of Al, presumably from the LAO substrate, were detected [see the second spectrum from the top in Fig. 16(h)]. This indicates that the composition of the area between any two nanodots has severely deviated from the stoichiometric composition of the original YBCO. The question is, how does this occur? Fig. 16. (a) SEM images show the surface morphology of YBCO thin films at various laser fluences (a) F = 0 mJ/cm 2 , (b) F = 210 mJ/cm 2 , (c) F = 320 mJ/cm 2 , (d) F = 530 mJ/cm 2 , (e) F = 260 mJ/cm 2 . (f) AFM image of (e). (g) X-ray diffraction patterns of YBCO thin films at various laser fluences corresponding to (a)-(e). (h) EDS spectra show the composition of area 1 and area 2 in (d) and (e). Due to the laser pulses, the transient increase in temperature, ΔT, can be estimated using the following relation ΔT = W / CV, where W is the pulse energy, C is the heat capacity, and V is the illuminated volume. For YBCO at 300 K using C = 2.86×10 6 J/m 3 K [derived from the Debye heat capacity and the Debye temperature of YBCO was obtained from ref. (Stupp & Lasers – Applications in Science and Industry 18 Ginsberg, 1989)], V = 1.14×10 -14 m 3 (the absorption length ~ 300 nm), and W on the order of 0.1 mJ (which is assumed to be totally absorbed by YBCO). ΔT is approximately 3000 K. This increase in temperature, in principle, will lead to massive global melting of a thin layer beneath the surface of YBCO thin films. Thus, a more random pattern would be expected when re-solidified. However, due to the interference induced by the inhomogeneous input energy, the YBCO in melted phase initially forms ripples according to the interference pattern which pushes the YBCO to the line of destructive interference. This interference pattern also leads to a periodic distribution of the fluctuations in temperature, ΔT, which happen to be higher than the boiling point of Ba [1897 K (Thompson & Vaughan, 2001)] along the line of constructive interference and lower than the boiling point of Ba [1897 K (Thompson & Vaughan, 2001)] along the line of destructive interference. As a result, in the regions of the constructive interference most Ba was vaporized, while in the destructive regions the Ba remained. Moreover, due to the surface tension and heterogeneous nucleation on the surface of the substrate, the melted YBCO along the lines of destructive interference aggregates to form nanodots in a periodic fashion, as shown in Fig. 16(b), 16 (e), and 16(f). These results suggest that, by using single-beam femtosecond laser irradiation, it is possible to fabricate a self-organized array of YBCO nanodots with most of the crystallinity and superconducting properties remaining intact, provided proper control of irradiation fluence is practiced. This technique could potentially be applied to the fabrication of microwave filter devices with array structure or the weak-link Josephson junction arrays. Fig. 17. Resistance versus temperature curve measured prior to femtosecond laser irradiation (F = 0 mJ/cm 2 ) and the magnetization versus temperature curve measured at 10 Oe after femtosecond laser irradiation, with various fluences corresponding to the Fig. 16 (c), 16(d), and 16(e), respectively. Nanoparticles and Nanostructures Fabricated Using Femtosecond Laser Pulses 19 Finally, as the fluence reached ≧ 320 mJ/cm 2 , irregular, disordered patterns were observed on the surface of the LAO substrate, as shown in Fig. 16(c) and Fig. 16(d). The characteristic XRD peaks of the (001)-YBCO films deteriorated significantly [Fig. 16(g)], indicating that the crystalline structure of YBCO had been destroyed by the higher laser fluence. EDS analysis [Fig. 16(h)] also shows that Ba was absent in both area 1 and area 2, marked in Fig. 16(d). In area 2, even the composition of Y is absent in the EDS spectrum. Using the previous estimation with W ≧ 0.12 mJ (fluence ≧ 320 mJ/cm 2 ), ΔT ≧ 3700 K was obtained, which is higher than the boiling point of Ba [1897 K (Thompson & Vaughan, 2001)] at the positions of both constructive and destructive interference, but only higher than the boiling point of Y [3345 K (Thompson & Vaughan, 2001)] at the position of constructive interference. In this case, the aggregation of melted YBCO becomes more disordered and the stoichiometric composition is more severely influenced, leading to the loss of crystalline integrity and superconductivity in the remaining residue of the original YBCO film. 4. Conclusions In this chapter, we demonstrated a simple, rapid means to obtain the hexagonal ZnSe nanoparticles, YBCO ripples, and dot structures. In the fabrication of ZnSe nanoparticles, while femtosecond laser pulses were focused on the surface of ZnSe wafers in air and the ablated plume cannot expand as rapidly as plumes would in a vacuum chamber which causes an instantaneous high-energy, high-pressure region around the focal point of the laser; meanwhile, a large amount of spherical-shape ZnSe nanoparticles with an average diameter of 16-22 nm (depending on the laser fluence) forms on the surface of the wafer. During the formation of ZnSe nanoparticles, the structural phase further changes from cubic to metastable hexagonal phase due to the ultrahigh localized ablation pressure caused by the rapid injection of high laser energy within a femtosecond time scale. For the generation of ripple and dot structures, we have systematically studied the surface morphology of YBCO thin films under a single-beam and a dual-beam fs laser irradiation. The generation of ripple and dot periodic structures was determined by the applied laser fluence, number of pulses, and polarization of the laser. The period and orientation of ripples, and even the size and density of dots can be controlled by these parameters. With lower laser fluence, the (001)-YBCO film turns into (001)-ripple or dot arrays with superconductivity remaining nearly intact. These rippled (or dotted) structures and superconductivity, however, were rapidly destroyed with higher fluence. These results may be applied to enhance the critical current of YBCO thin films and the fabrication of the microwave filter devices with array structures or the weak-link Josephson junction arrays. The present results clearly demonstrate that the femtosecond laser, in addition to its crucial role in studying the ultrafast dynamics of matter, they can also serve as a new avenue for engineering materials and structures into their surfaces at a nanometer scale. 5. Acknowledgments The author would like to express his sincere appreciation and gratitude to his collaborators and colleagues, Ms. H. I. Wang, Mr. W. T. Tang, Ms. C. C. Lee, and Mr. L. W. Liao, Profs. T. Kobayashi, K. H. Wu, J. Y. Juang, J Y. Lin, T. M. Uen, C. S. Yang. This work was supported by the MOE-ATU program at NCTU and National Science Council of Taiwan, under Grant No. NSC 98-2112-M-009-008-MY3. Lasers – Applications in Science and Industry 20 6. References Amoruso, S.; Bruzzese, R.; Spinelli, N.; Velotta, R.; Vitiello, M.; Wang, X.; Ausanio, G.; Lannotti, V. & Lanotte, L. (2004). Generation of Silicon Nanoparticles via Femtosecond Laser Ablation in Vacuum. 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Origin of Laser-Induced Near- Subwavelength Ripples: Interference between Surface Plasmons and Incident Laser. ACS Nano, Vol.3, No.12, (November 2009) pp. 4062-4070, ISSN 1936-0851 Jiang, Y.; Meng, X. M.; Yiu, W. C.; Liu, J.; Ding, J. X.; Lee, C. S. & Lee, S. T. (2004). Zinc Selenide Nanoribbons and Nanowires. The Journal of Physical Chemistry B, Vol.108, No.9, (March 2004) pp. 2784-2787, ISSN 1520-6106 Jia, T. Q.; Zhao, F. L.; Huang, M.; Chen, H. X.; Qiu, J. R.; Li, R. X.; Xu, Z. Z. & Kuroda, H. (2006). Alignment of Nanoparticles Formed on the Surface of 6H-SiC Crystals Irradiated by Two Collinear Femtosecond Laser Beams. Applied Physics Letters, Vol.88, No.11, (March 2006) pp. 111117, ISSN 0003-6951 Jia, X.; Jia, T. Q.; Zhang, Y,; Xiong, P. X.; Feng, D. H.; Sun, Z. R.; Qiu, J. R. & Xu, Z. Z. (2010). Periodic Nanoripples in the Surface and Subsurface Layers in ZnO Irradiated by Femtosecond Laser Pulses. Optics Letters, Vol.35, No.8, (April 2010) pp. 1248-1250, ISSN 0146-9592 [...]... to handle hot spots in the beams 28 Lasers – Applications in Science and Industry 4 Depositing high LIDT coatings at Sandia’s large optics coating operation Coating large optics goes hand in hand with large vacuum coating chambers In Sandia’s case, the coating chamber is 2. 3 m x 2. 3 m x 1.8 m in size and opens to a Class 100 clean room equipped for handling and cleaning the large optics for coating... coatings of a diagnostic beamsplitter The coatings are for non-normal angle 1 Contract Associate to Sandia (JB with Sandia Staffing Alliance; DK with LMATA Government Services) 24 Lasers – Applications in Science and Industry of incidence (AOI), and the designs take into account behaviors of both S and P polarization (Spol and Ppol) electric field intensities resulting from interference of forward and. .. and “short” means sub-ns class pulses The resulting intensities of these laser pulses are typically terawatt (TW) to PW and even higher Focusing of the beams leads to corresponding fluences of 1016 W/cm2 to 1019 W/cm2 and beyond, approaching 1 022 W/cm2, depending on the particular laser system and on the achievable minimum focal spot size Aberrations prevent focusing in the diffraction limit, so minimizing... the area in front of the coater from the rest of the Class 100 area, shown in Fig 2, in which optics undergo cleaning and preparation for coating Another such measure is to handle optics in preparation for coating and to load them into the chamber using special tooling and techniques that protect the surfaces undergoing coating from exposure to the non-Class 100 conditions in front of and inside the... Z.; Zhang, X T.; Wong, C C & Hark, S K (20 06) Wurtzite ZnSe nanowires: growth, photoluminescence, and single-wire Raman properties Nanotechnology, Vol.17, No .22 , (November 20 06) pp 5561-5564, ISSN 0957-4484 22 Lasers – Applications in Science and Industry Sakabe, S.; Hashida, M.; Tokita, S.; Namba, S & Okamuro, K (20 09) Mechanism for SelfFormation of Periodic Grating Structures on a Metal Surface by a... some coatings as a means of mitigating stress mismatch between the coating and substrate We continue with details of preparation of large optics for coating, including the polishing and washing and cleaning of the substrate surfaces, in ways that insure the highest LIDTs of coatings on those surfaces We turn next to LIDT tests with nanosecond and sub-picosecond class laser pulses while emphasizing the... on-going research Gains in energy capacity per pulse of a given laser system due to improvements in the LIDTs of optics and coatings can be significant, amounting to factors of 2 or more, but usually less than 10 As mentioned, laser-induced aberrations within gain media and optics undermine the achievement of ultra-high intensities by causing distortion of the beam’s wave front and corresponding decrease... of coating design in more detail in this chapter Regarding the polishing and preparing of optics for coating, we have demonstrated in the case of an AR coating that using one combination of polishing compound and wash preparation for the substrate prior to coating over another can lead to an improvement by a factor of 2 in the laser damage threshold of the coating, and hence the energy capacity per... the coating chamber needs to be ~ 1 – 2 X 10-6 Torr in order to insure contamination free conditions for the deposition process Fig 2 Sandia’s Class 100 clean room for washing and preparing large optics for coating Achieving high LIDT coatings depends not only on use of coating materials with high resistance to laser damage, but also on the methods of preparing the substrate surfaces for coating and on... hot 26 Lasers – Applications in Science and Industry spots that are not uncommon in the cross section of high intensity laser beams Fused silica and BK7 are among the most laser damage resistant optical grade glasses (Wood, 20 03), and Nd:Phosphate Glass and Ti:Sapphire are laser gain media that also exhibit high fluence thresholds for laser-induced damage (Wood, 20 03) and at the same time afford some . handle hot spots in the beams. Lasers – Applications in Science and Industry 28 4. Depositing high LIDT coatings at Sandia’s large optics coating operation Coating large optics goes hand. Services) Lasers – Applications in Science and Industry 24 of incidence (AOI), and the designs take into account behaviors of both S and P polarization (Spol and Ppol) electric field intensities. hand in hand with large vacuum coating chambers. In Sandia’s case, the coating chamber is 2. 3 m x 2. 3 m x 1.8 m in size and opens to a Class 100 clean room equipped for handling and cleaning